In a conventional optical disc system (i.e., CD or DVD), to move a laser beam from one track to another track of the disc, a tracking actuator is used to move the lens across these tracks. Lens position information during this motion is crucial for motion feedback control to move the lens from a current track position to the target track position.
Referring to FIG. 1, an optical disc system 10 is shown. To control the position of a laser beam 20 on the disc 22, the position of the objective lens 24 of the Optical Pickup Unit (OPU) 25 related to the tracks 26a-26n on the optical disc 22 is controlled by a tracking actuator. The OPU 25 is a device configured to emit a laser beam and pick up the optical reflection of the beam and convert such reflections to electrical signals.
Conventional approaches use a tracking error signal TE along with another signal, often called a ripple signal, to obtain the position of the lens 24 in relation to the tracks 26a-26n of the optical disc 22. The ripple signal is about 90 degrees different (either faster or slower) in phase than the signal TE (depending on the direction the lens 24 moves).
Using another signal beside the signal TE uses more hardware and/or firmware support to create and calibrate this additional ripple signal. In some types of media, the ripple signal is very difficult to obtain and not always reliable. The process of obtaining lens position information becomes more complicated and even not accurate. It is very difficult to create a reliable ripple signal, especially in different types of optical media or in a disc that is partially filled with data and is partially blank. The poor quality of the ripple signal makes the process of obtaining lens position information more complicated and even not accurate. As a result, feedback control for tracking the actuator is also difficult to implement. In a worst case, a poor ripple signal can make the control system unstable and cause a failure in lens motion.
It would be desirable to provide a method and/or apparatus to extract the lens position information using only the tracking error signal (TE) during motion control. It would also be desirable to implement such a system that is simple and allows a closed loop feedback control during movement of a laser beam across the tracks of an optical disc.